I. Field of the Invention
This invention relates to an arrangement of busbars by which current is conveyed from an aluminum electrolytic cell. More particularly, at least in certain aspects, the invention relates to an arrangement of busbars for cells arranged transversely in a row whereby current is conveyed from one cell to the cell next downstream in the row.
II. Description of the Prior Art
A typical aluminum electrolytic cell is generally rectangular having longitudinal and transverse axes and comprises a pot containing a molten cryolite-based electrolyte at a temperature of 905.degree.C. to 980.degree.C. Dipping into this electrolyte are carbonaceous anodes suspended by anode rods from generally two anode beams extending longitudinally of the cell. The potlining includes a carbonaceous floor which constitutes part of the cathode structure of the cell. Embedded in the carbonaceous floor are steel collector bars which extend transversely of the cell and are spaced longitudinally along it. Aluminum metal is formed by electrolysis as a molten pool (pad) of metal overlying the cell floor beneath the layer of molten electrolyte, from where it is periodically tapped. Alumina is added to and dissolved in the electrolyte as electrolysis proceeds, and oxides of carbon are removed.
These cells are arranged transversely in rows with the electric current being passed from the cathode of an upstream cell to the anode of the cell next downstream.
By "arranged transversely in rows" we mean that the cells (which are usually identical) are arranged with their transverse (short) axes parallel to, and indeed normally coincident with, the axis of a row, with each cell having a downstream side (adjacent the next downstream cell in a row) and an upstream side. The collector bars embedded in the floor of a cell extend parallel to the axis of the row and terminate at bar ends, half on the downstream side of the cell and the other half on the upstream side. Busbars and anode risers positioned outside the cell are used to carry the electric current from these collector bar ends to the anode beams of the cell next downstream.
The design of these busbars and risers is subject to various criteria. One is that they should be positioned so as to minimize the magnetic field induced in the cell, particularly the vertical component thereof. The vertical component of the induced magnetic field interacts with the horizontal component of the electric currents in the molten metal pad giving rise to horizontal forces which can affect different regions of the metal pad in different ways causing metal motion, humping of the metal surface and wave formation. These disturbances make it necessary to maintain a larger anode to cathode distance than would otherwise be desirable, which in turn increases the internal resistance of the cell.
The present tendency to build larger cells and operate them at higher current densities aggravates these problems and gives rise to further problems, for example the cost and difficulty of housing and providing adequate access to long rows of large cells.
These problems are known and various busbar arrangements have been proposed to overcome them. One type of arrangement involves passing some of the electric current from the upstream collector bars through busbars extending round the ends (i.e. adjacent the short sides) of the cell; and passing the remaining current from the upstream collector bars through busbars extending underneath the cell. By such arrangements, the vertical component of the induced magnetic field can be minimized and evened out over various regions of the cell. Arrangements of this kind are described, for example, in U.S. Pat. No. 3,415,724; U.S. Pat. No. 4,313,811; U.K. Pat. No. 1,032,810; USSR Authors Certificate No. 434 135; and Canadian Pat. No. 1,061,745.
U.S. Pat. No. 4,474,611 describes a modification of such an arrangement in which asymmetry is introduced into the distribution of current beneath and around the ends of the cells. This is achieved by dividing the upstream collector bars into three groups which are asymmetric with regard to the transverse cell axis, connecting the central (or approximately central) group to busbars passing beneath the cell, and connecting the end groups to busbars passing around the respective ends of the cell (with, in one embodiment, some of the collector bars of one end group being connected to busbars passing around the "wrong" end of the cell to achieve asymmetry). The advantage of this is stated to be a compensation for the magnetic effects from neighbouring rows of cells.
While the magnetic effects induced by neighbouring rows of cells might be of concern in those installations having two or more closely spaced rows, the magnetic effects of neighbouring cells within one row are generally more important to cell operation when the rows are appropriately spaced apart.